Strabismus is a condition in which the eyes are not properly aligned with each other.[1] It typically involves a lack of coordination between the extraocular muscles that prevents bringing the gaze of each eye to the same point in space and preventing proper binocular vision, which may adversely affect depth perception. Strabismus can be either a disorder of the brain coordinating the eyes or a disorder of one or more muscles, as in any process that causes a dysfunction of the usual direction and power of the muscle or muscles.

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During eye examinations, ophthalmologists, orthoptists, and optometrists typically use a cover test to aid in the diagnosis of strabismus. If the eye being tested is the strabismic eye, then it will fixate on the object after the "good" eye is covered, as long as the vision in this eye is good enough.
If it is the "good" eye, there will be no change in fixation, as it is already fixated.
Depending on the direction that the strabismic eye deviates, the type of tropia or phoria may be assessed.

A simple screening test for strabismus is the Hirschberg test. A flashlight is shone in the patient's eye. When the patient is looking at the light, a reflection can be seen on the front surface of the pupil. If the eyes are properly aligned with one another, the reflection will be in the same spot of each eye. Therefore, if strabismus is present, the reflection from the light will not be in the same spot of each eye.

Strabismus may be classified as unilateral, bilateral, or alternating based on whether one eye or both eyes are affected. A unilateral strabismus will consistently have the same eye 'wandering'. Bilateral strabismus is a condition where both eyes are squinting at the same time; either convergently or divergently (both are subtypes of Concomitant strabismus). Finally, an alternating strabismic patient can fixate on a target object with either eye. In this case either eye may be askew while the opposite eye is focused on the target. Optometrists use the cover-uncover test to diagnose various types of tropias, which is a medical term for strabismus. [1]

As with other binocular vision disorders, the primary therapeutic goal for those with strabismus is comfortable, single, clear, normal binocular vision at all distances and directions of gaze.[2]

Whereas amblyopia, if minor and detected early, can often be corrected with use of an eyepatch on the dominant eye and/or vision therapy, the use of eyepatches is unlikely to change the angle of strabismus. Advanced strabismus is usually treated with a combination of eyeglasses or prisms, vision therapy, and surgery, depending on the underlying reason for the misalignment. Surgery attempts to align the eyes by shortening, lengthening, or changing the position of one or more of the extraocular eye muscles and is frequently the only way to achieve cosmetic improvement. Glasses affect the position by changing the person's reaction to focusing. Prisms change the way light, and therefore images, strike the eye, simulating a change in the eye position.

Early treatment of strabismus and/or amblyopia in infancy can reduce the chance of developing amblyopia and depth perception problems. Eyes that remain misaligned can still develop visual problems. Although not a cure for strabismus, prism lenses can also be used to provide some comfort for sufferers and to prevent double vision from occurring.

In adults with previously normal alignment, the onset of strabismus usually results in double vision (diplopia).

When strabismus is congenital or develops in infancy, it can cause amblyopia, in which the brain ignores input from the deviated eye. Strabismus can lead to a permanent weakening of vision in the strabismic eye called amblyopia sometimes referred to as lazy eye. The appearance of strabismus may also be a cosmetic problem. One study reported that 85% of adult strabismus patients "reported that they had problems with work, school and sports because of their strabismus." The same study also reported that 70% said strabismus "had a negative effect on their self-image."[6]

Pseudostrabismus is the false appearance of strabismus. It generally occurs in infants and toddlers whose bridge of the nose is wide and flat, causing the appearance of strabismus. With age, the bridge of the child's nose narrows and the folds in the corner of the eyes go away. To detect the difference between pseudostrabismus and strabismus, a Hirschberg test may be used.

Strabismus can be an indication that a cranial nerve has a lesion. Particularly Cranial Nerve III (Occulomotor), Cranial Nerve IV (Trochlear) or Cranial Nerve VI (Abducens). A strabismus caused by a lesion in either of these nerves results in the lack of innervation to eye muscles and results in a change of eye position. A strabismus may be a sign of increased intracranial pressure, as CN III is particularly vulnerable to damage from brain swelling.

Strabismus is often incorrectly referred to as "lazy eye" (which in fact refers to the associated condition Amblyopia). It is also known as heterotropia, squint, "one eye in York and the other in Cork," "crossed eye," "cockeye," "wandering eye," "weak eye," "wall eye."